Hydrophilic gels that contain up to 95 percent water are known. Some of these hydrogels were prepared by irradiation crosslinking and exhibited smooth non-tacky surfaces. More recently, hydrogels having interesting surface characteristics, such as adhesiveness, tackiness or non-stringiness, have been described. These hydrophilic gels can be manufactured into sheets and a number of applications have been disclosed which utilize these materials. For example, hydrogel-based wound dressings are available which permit water soluble drugs to migrate though the hydrogel film without disruption of the film's bacterial barrier properties (See, U.S. Pat. No. 3,419,006). Conventional hydrogels can impart a cooling sensation when applied to the skin. This property is desirable in applications that include contact with inflamed or sensitive areas of the body. However, conventional hydrogel dressings do not readily adhere to exposed wound tissue.
Hydrogel sheets adapted for use in medical electrode applications are also known. Manufacturers of these sheets include Promcon, a Division of Medtronic, Inc. (Brooklyn Center, Minn.); Valleylab, Inc., a Division of Pfizer, Inc. (Boulder, Colo.) Biostim, Inc. (Princeton, N.J.); Lectec Corp. (Eden Prairie, Minn.); and Conmed (Utica, N.M.).
Numerous patents disclose hydrophilic gels or medical electrodes that employ them. For instance, U.S. Pat. No. 4,989,607 describes highly conductive hydrogels comprised of a cohesive uniform mixture of poly(vinyl pyrrolidone) ("PVP"), a viscosity-enhancing hydrophilic polymer, and an effective amount of an electrolyte. These hydrogel compositions display non-stringy adhesive cohesive characteristics. However, these materials dry out quickly and must be replaced frequently to maintain the tackiness required to keep the electrode in place.
U.S. Pat. No. 4,904,247 describes a pressure-sensitive hydrophillic laminate composite made up of contiguous layers of a tacky, pressure sensitive hydrophillic polymer blend and a non-tacky hydropic polymer blend. This patent describes a composite structure of uncrosslinked polymer mixtures which require plasticizers to achieve adhesion.
Although U.S. Pat. No. 4,904,247 describes uncrosslinked structures, hydrogels have been described in the art as three-dimensional structures containing crosslinked water soluble polymer and both bound and entrapped water. See N. A. Peppas and A. G. Mikos, "Preparation Methods and Structure of Hydrogels," Hydrogels in Medicine and Pharmacy: Volume I Fundamentals, N. A. Peppas, Ed., CRC Press, Inc., Boca Raton, Fla., 1986, Pg. 2. Therefore, by accepted art definitions, the composite structure of U.S. Pat. No. 4,904,247 is not a hydrogel. Absorption of fluids into the gel therefore would tend to dissolve the uncrosslinked, water soluble polymers thereby leading to eventual mechanical failure of a device that employs the gel compositions of U.S. Pat. No. 4,904,247.
U.S. Pat. No. 4,860,754 to Sharik describes a material that possesses adhesive, cohesive, elastomeric and conductive properties and which is composed of a plasticizer, a high molecular weight water-soluble polymer, uncrosslinked PVP as the tackifier and an electrolyte dopant. It is known that materials containing about 20% by weight of water are less subject to drying out, but Sharik found that hydrogels of uncrosslinked PVP which contain up to 40% by weight of water retain both their electrical properties, as well as their adhesive and cohesive properties.
Although examples of hydrogels and medical electrode assemblies are known, these prior materials suffer from one or more characteristics that limit or, in some cases, preclude their utility as adhesive wound dressings, drug delivery systems, medical electrodes or the like. In particular, these prior materials either dry out rapidly, become brittle when cooled, or have adhesive surface characteristics that are lost upon the slightest over-exposure to crosslinking radiation. Also, none of the prior disclosures describe the flexible, non-stringy, adhesive, cold use characteristics of the hydrogels of the present invention. Moreover, none embody the unique combination of such desirable surface and low-temperature pliability properties with the extended in-use moisture retention characteristics of the present invention. Indeed, none are, at the same time, amenable to sterilization by high energy radiation.